Transparent ceramics have unique properties enabling applications no other materials can and magnesium aluminate spinel has been at the forefront of their development. However, the processing-property relationships of transparent spinel fabrication, in particular interfaces, which often govern densification and influence final properties, remain poorly understood. In this light, the interfaces of transparent spinel were studied in detail; grain boundaries and surfaces of transparent spinel compacts made using powders with different stoichiometries and impurity and LiF additive contents, and using a variety of densification methods, were examined using optical microscopy, scanning and transmission electron microscopy, various chemical spectroscopy methods, atomic-force microscopy, and electrochemical impedance spectroscopy. In turn, interface properties were related to starting powders, processing, and mechanical and opto-electronic properties.
Differences in stoichiometry and impurity and LiF content altered the free energy, diffusion characteristics, lattice parameter, and mechanical, optical, and electronic properties of surfaces and grain boundaries. The interface energies of astoichiometric compositions were more conducive to densification, but enhanced interface transport in Al2O3-rich compositions caused coarsening that precluded densification by pressureless sintering while assisting it with added pressure. Impurities and carbon contamination from thermal treatment were the main sources of optical scatter and absorption and they also affected conductivity. Dielectric properties were found to be a complex function of microstructure, stoichiometry, impurities, and additives, and higher conductivity compared to single crystals was attributed to conductance along impurity-rich magnesium-depleted boundaries. Lastly, compact-scale stoichiometry, impurity, and additive content gradients resulted in associated microstructure and property gradients.
The LiF sintering additive had a profound, complex, and multi-faceted effect. Fluorine reacted with impurities to form volatile species, which were removed by careful processing to render transparency, and it reacted with spinel to form magnesium fluoride and oxide, altering stoichiometry. Whereas, lithium incorporated into the lattice and altered interface properties, resulting in; (i) a shift towards Al2O3-rich composition, (ii) the removal of impurities, (iii) accentuation of grain-boundary and compact-scale stoichiometry and microstructure gradients, (iv) enhanced interface transport and grain growth, (v) reduced densification temperature and enhanced densification with pressure, (vi) coarsening and inhibited densification without pressure, (vii) grain-boundary embrittlement, (viii) grain-boundary optical scatter, (ix) opacity when processing trapped LiF and its by-products, and (x) lower conductivity due to conductance-inhibiting point defects and decreased grain-boundary area and impurities.
The work demonstrated how interfaces were affected by starting powders and processing parameters and in turn how this affected microstructure, fracture behavior, and opto-electronic properties. The findings shed light on many intricacies of transparent spinel fabrication, thus giving guidance on how to tailor processing and microstructure to yield improved bulk properties
